Because of being excellent in toughness and flame retardancy, cellulose acylate films have been employed in photographic supports and various optical materials. In recent years, in particular, cellulose acylate films are frequently employed as optically transparent films for liquid crystal display devices. Owing to the high optical transparency and high optical isotropy, cellulose acylate films are favorable as optical materials for devices with the use of polarization such as liquid crystal display devices. Therefore, cellulose acylate films have been employed as optically compensatory film supports whereby display in looking from an angle can be compensated (viewing angle compensation).
A polarizing plate, which is one of members constituting a liquid crystal display device, is constructed by bonding a polarization film-protecting film to at least one side of a polarization film. In general, a polarization film is obtained by dyeing a stretched polyvinyl alcohol (PVA)-based film with iodine or a dichroic dye. Such a polarization film-protecting film should be excellent in optical isotropy and the characteristics of a polarizing plate largely depend on the optical characteristics of the polarization film-protecting film. As the polarization film-protecting film, therefore, cellulose acylate films, in particular, triacetyl cellulose films which can be bonded directly to PVA are employed in may cases.
Before bonding a protecting film to a polarization film, the bonding face of the protecting film is subjected to a surface-treatment such as a hydrophilicating treatment in some cases so as to enhance the adhesiveness to the polarization film. As the hydrophilicating treatment, use is frequently made of an alkali saponification treatment and it has been also proposed to employ a plasma treatment, a corona treatment and so on therefor (see, for example, JP-A-2002-328224 and JP-A-2000-356714).
In liquid crystal display devices in these days, it is more strongly required to improve viewing angle characteristics. Thus, optically transparent films to be used as a polarization film-protecting film, an optically compensatory film support, etc. should have improved optical isotropy. To be optically isotropic, it is important to have a small retardation value represented by the product of the birefringence and thickness of an optical film. To improve the display in looking from an angle, it is particularly needed to lower not only the in-plane retardation value (Re) but also the thickness direction retardation value (Rth). More specifically speaking, it is needed that, in the case of evaluating the optical characteristics of an optically transparent film, Re measured in plane is a small value and Re shows no change even though the measurement angle is varied.
Although there have been cellulose acylate films having a small in-plane retardation Re, a cellulose acylate film having a small change in Re depending on angle, i.e., having a small Rth can be hardly produced. Thus, there have been proposed optically transparent films with little angle-dependent change in retardation with the use of a polycarbonate-based film or a thermoplastic cycloolefin film as a substitute for a cellulose acylate film (see, for example, JP-A-2001-318233 and JP-A-2002-328233; examples of commercially available products being ZEONOR (manufactured by ZEON CORPORATION) and ARTON (manufactured by JSR)). In the case of using as a polarization film-protecting film, however, these optically transparent films suffer from a problem in the bonding properties to PVA. Moreover, there still remains another problem that the optical characteristics in the entire film face are uneven. To overcome these problems, it is efficacious to further lower the optical anisotropy.
In producing a cellulose acylate film, it has been a practice to add a compound called a plasticizer to thereby improve the film-forming performance. Examples of the plasticizer include phosphoric acid triesters such as triphenyl phosphate and biphenyldiphenyl phosphate and phthalic acid esters and so on (see, for example, Purasuchikku Zairyo Koza, vol. 17, Nikkan Kogyo Shinbun, Ltd., Senisokei Jushi, p. 121 (1970)). It is known that some of these plasticizers have an effect of lowering the optical anisotropy of a cellulose acylate film. For example, specific fatty acid esters are disclosed (see, for example, JP-A-2001-247717). However, these known compounds can only insufficiently lower the optical anisotropy of a cellulose acylate film.
As a method of producing a biaxial optically compensatory film to be used in liquid crystal display devices in recent years, there has been proposed a method which comprises providing a thin layer having a high-molecular weight polymer as the main component on a supporting material and orienting the high-molecular weight polymer by either a stretching treatment, a shrinking treatment or both of them to thereby give an optical film having a desired retardation (see JP-A-2003-315541, JP-A-2001-344856, JP-A-2004-46097 and JP-A-2004-78203). In this method, the supporting material should have small Re and Rth after the stretching or shrinking treatment too. However, supporting materials proposed hitherto have large retardation after the stretching or shrinking treatment or polarizing plates constructed by using the thus obtained optically compensatory films as protecting films suffer from some problems in the bonding properties or durability.